Hydrocarbon upgrading and conversion processes such as catalytic cracking, pyrolysis, hydroprocessing, reforming, and the like can be utilized for producing mixtures comprising molecular hydrogen, methane, and other hydrocarbons. It is generally desirable to separate methane and molecular hydrogen from the mixture, e.g., utilizing boiling-point separation. For example, vapor mixtures comprising molecular hydrogen and methane can be exposed to a relatively low temperature in order to condense a stream comprising at least a portion of the mixture's methane. The condensed methane can then be separated from the molecular hydrogen, which remains in the vapor phase. Such separations can be carried out in a system of integrated heat exchangers and separation drums, e.g., a conventional cold box.
One difficulty encountered in such low-temperature separations involves the accumulation in a cold box or other low-temperature region of gums and/or salts of compounds comprising oxygen and nitrogen (“NOx”). NOx gums, which result from the reaction of NOx with hydrocarbons, have been observed to react in an uncontrolled manner (e.g., explosively) at sub-ambient temperatures, such as those temperatures encountered when a cold box is warmed for maintenance, shutdown, unplanned temperature excursion (resulting, e.g., from process gas compressor failure, refrigeration system failure, etc.), and the like. The accumulation of NOx salts represents a similar concern because these can also react in an uncontrolled manner, albeit at higher temperature than that of NOx gums (e.g., at or near ambient temperatures), and can contribute to the total energy released during an uncontrolled reaction initiated by NOx gums.
The accumulation of NOx gums and salts can be lessened by removing NOx compounds downstream of a hydrocarbon upgrading or conversion process but upstream of low-temperature region utilized for methane condensation and separation. For example, U.S. Pat. No. 5,955,045 discloses a method for the selective chemisorption of NOx compounds, such as those found in the waste gas of a fluidized catalytic cracking unit. In an example, the patent discloses an NO removal rate of 85% after an operating time of more than 130 hours by exposing a mixture comprising methane, molecular hydrogen, and 20 mol ppb of NO to a catalyst comprising MnO2 on Al2O3 at a space velocity of about 4000 h−1. Utilizing relatively low-capacity adsorbents at a relatively low space velocity provides a relatively long run length before adsorbent regeneration is needed.
It is desired to improve NOx-removal technology, e.g., by utilizing higher space velocities and/or NOx removal agents having increased capacity, in order to remove an even greater amount of NOx removed from mixtures containing methane, molecular hydrogen, and optionally, water and other saturated and/or unsaturated hydrocarbons.